An ultrasonic sensor has been conventionally used, which can determine the presence or absence of an object or measure a distance between the sensor and an object by using a single ultrasonic vibrator to transmit an ultrasonic signal and receive a reflected signal. Such an ultrasonic sensor cannot correctly receive the reflected signal until reverberation (i.e., terminal voltage swing that continues to some extent after driving signals for the ultrasonic vibrator are stopped), produced after the transmission of the ultrasonic signal, is stabilized. Therefore, the conventional ultrasonic sensor has to take a measure against the reverberation, such as masking the received signal until a predetermined period of time elapses after transmitting the ultrasonic signal, in order to prevent the reflected signal from being incorrectly detected.
However, the configuration of masking the received signal over the predetermined period of time after transmitting the ultrasonic signal has a problem in that the reflected signal cannot be correctly detected if the reflected signal is returned immediately after transmitting the ultrasonic signal (that is, it is difficult to make determination on the presence or absence of a near object or measure a distance between the sensor and the near object).
In the related art, a technique is disclosed for eliminating reverberation by driving an ultrasonic vibrator with a reverberation inhabitation signal having an opposite phase to an ultrasonic signal for a predetermined period corresponding to a half cycle of the ultrasonic signal immediately after stopping the ultrasonic signal (i.e., by setting a final pulse width of the ultrasonic signal to be twice as large as normal). However, since the ultrasonic vibrator has its own inherent reverberation property, the conventional technique cannot obtain a sufficient reverberation inhabitation effect in some cases.